Optical transmission system and optical coupler/branching filter

ABSTRACT

The objective is an optical transmission unit capable of coupling and branching signal lights of individual wavelengths at a data rate of 10 Gbits/s. In order to achieve the objective, DCFs are provided and proper values of dispersion compensation are given, so that different characteristics depending on the respective wavelengths to be coupled, split and pass through without being split are obtained. The signal lights are amplified by a plurality of low-excitation optical amplifiers to regain their light levels which are weakened due to using an optical coupler, branching filter and DCF together, thereby enabling a long-distance transmission.

BACKGROUND OF THE INVENTION

The present invention relates to a transmission system and an opticalsplitting unit, and particularly to a wavelength division multiplexing(WDM) transmission system for transmitting signal lights of differentwavelengths and an optical coupler/branching filter used for the system.

The recent growing traffic of data attributable to the prevalence of theInternet and the like necessitates the increase in the transmissioncapacity of an optical fiber cable. WDM and speed-up of transmission arepossible schemes for the increase of transmission capacity.

The WDM system is designed to transmit a number of signal lights ofdifferent wavelengths based on multiplexing through an optical fibercable. Based on the splitting and coupling of part of multiplexed signallights, it becomes possible for an optical fiber capable of transmittingthe signal lights not only between two places but among a number ofplaces.

There is proposed, for a data rate of 2.4 Gbits/s, a method of splittingand coupling part of wavelengths at once by disposing an opticalcoupler/branching filter formed of passive parts between opticalamplifiers. A technique relevant to this method is described in JapanesePatent Laid-open (Kokai) No. Hei 11-275007.

At a data rate of 10 Gbits/s which is four times the 2.4 Gbits/s, theinfluence of light dispersion on the transmission path is not negligibleand the compensation against dispersion is required. A techniquepertinent to the dispersion compensation is described in Japanese PatentLaid-open (Kokai) No. Hei 7-301831.

However, for the transmission at 10 Gbits/s on a transmission path ofusual optical fiber (non-dispersion shifted fiber: NDSF), it isdifficult for the above-mentioned conventional WDM technique to equipnecessary dispersion compensating fibers (DCF). The transmission at 10Gbits/s necessitates DCFs of different characteristics depending on therespective wavelengths to be coupled, split and passed through withoutbeing split.

Generally, the optical branching filter, optical coupler and DCF areoptical parts which suffer large losses, and therefore using the opticalcoupler/branching filter and DCF in series results in an increased loss,making a long distance transmission difficult.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical transmissionunit which is capable of performing the coupling and branching ofindividual wavelengths at a data rate of 10 Gbits/s.

In order to achieve the above objective, the inventive opticalcoupler/branching filter is designed to set proper values of dispersioncompensation depending on the state of transmission path.

The loss of signal light caused by using an optical coupler, an opticalbranching filter and DCF together is compensated by means of a pluralityof low-excitation optical amplifiers, thereby enabling a long-distancetransmission.

These and other objects and many of the attendant advantages of theinvention will be readily appreciated, as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a bidirectional optical transmission systemwhich is an embodiment of this invention;

FIG. 2 is a block diagram of a bidirectional optical transmission systemwhich is another embodiment of this invention;

FIG. 3 is a block diagram of a bidirectional optical transmission systemwhich is another embodiment of this invention;

FIG. 4 is a block diagram of an optical transmission unit which is anembodiment of this invention;

FIG. 5 is a block diagram of an optical transmission unit which isanother embodiment of this invention;

FIG. 6 is a block diagram of an optical transmission unit which isanother embodiment of this invention;

FIG. 7 is a block diagram of an optical branching filter which is anembodiment of this invention; and

FIG. 8 is a block diagram of a bidirectional optical transmission systemwhich is another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of this invention will be explained in detail withreference to the drawings.

FIG. 1 shows by block diagram a bidirectional optical transmissionsystem which is an embodiment of this invention. West station isequipped with an optical transmission unit 70 for the W-E (from West toEast) transmission, a transmission optical amplifier 3T, an opticalreceiving unit 90 for the E-W (from East to West) receiving, and areceiving optical amplifier 3R. East station is equipped with an opticaltransmission unit 70 for the E-W transmission, a transmission opticalamplifier 3T, an optical receiving unit 90 for the W-E receiving, and areceiving optical amplifier 3R. The West station and East stations cancommunicate with each other through a transmission path 7 at a data rateof 10 Gbits/s. Center station which is located between the West and Eaststations is equipped for the W-E transmission with a receiving opticalamplifier 3R, an optical coupling/branching & dispersion compensator 80,a transmission optical amplifier 3T, a split light signal receiver 90′,and a coupled light signal transmitter 70′. It also has the sameequipment for the E-W transmission.

The transmission path 7 is NDSF, which is common to all embodiments ofthis invention.

The optical transmission unit 70 includes 16 optical transmitters 71-1through 71-16 for signal light transmission of different wavelengthsλ1-λ16 (λ1<λn<λ16, n=2-15), and a wavelength multiplexer 6 whichwavelength-multiplexes the signal lights for transmission. The opticalreceiving unit 90 includes a wavelength demultiplexer 9 whichdemultiplexes a wavelength-multiplexed light signal being transmittedover the transmission path 7 into signal lights of the wavelengthsλ1-λ16, and transmits the demultiplexed signal lights to 16 opticalreceivers 91. The multiplexed signal lights being transmitted over thetransmission path 7 does not necessarily include all of the wavelengths.Each optical receiver 91 has a wide receiving wavelength band to bereceptible of any wavelength. DCFs (not shown) are placed between thewavelength multiplexer 6 of optical transmission unit 70 and the sendingoptical amplifier 3T, and between the wavelength demultiplexer 9 ofoptical reception unit 90 and the receiving optical amplifier 3R.

The optical coupling/branching & dispersion compensator 80 includes aDCF 21 through which a split wavelength and wavelengths passing throughan optical coupling/branching unit 100 pass, an optical branching filter4 which branches a signal light of wavelength λ1, a DCF 22 through whichthe wavelengths passing through the optical coupling/branching unit 100pass, a DCF 23 through which a wavelength λ1′ (λ1′=λ1) to be coupledfrom an optical transmission unit 70′ passes, and an optical coupler 5which couples the signal light from the DCF 23 to the signal light fromthe DCF 22.

The DCFs 21, 22 and 23 are determined to have dispersion values of −600ps/nm, −500 ps/nm and −400 ps/nm, respectively. Accordingly, the splitwavelength has a dispersion value of −600 ps/nm, the wavelengths passedthrough the optical coupling/branching unit 100 have a dispersion valueof −1100 ps/nm, and the coupled wavelength has a dispersion value of−400 ps/nm.

The DCFs have their dispersion values determined appropriately dependingon the dispersion values of the transmission path for receiving and thatfor transmitting. This affair is common to all embodiments of thisinvention. The optical branching filter 4 is of the filter type and theoptical coupler 5 is of the coupler type throughout the embodiments.

Based on this system configuration, it is possible for a splitwavelength, pass-through wavelengths and coupled wavelength to havedispersion compensation at proper dispersion values depending onindividual transmission distances, thereby enabling the high-speedtransmission at 10 Gbits/s.

Although λ1′ is equal to λ1 in this embodiment, it may be different,provided that it is not used between other transmitter and receiver inthe same segment. This affair is a common to all embodiments of thisinvention.

In this embodiment, the optical branching filter 4 and coupler 5 aredesigned to be a module of a printed circuit board having opticalconnectors (not shown), and it is used commonly as an opticalcoupling/branching unit 100 as will be explained in the following.

FIG. 2 and FIG. 3 show bidirectional optical transmission systems whichare other embodiments of this invention. In the figures, East and Weststations are omitted, and the same functional blocks are referred by thecommon reference numerals unless otherwise needed.

Center station of FIG. 2 is characterized by using one opticalcoupling/branching unit 100 for both the coupling of W-E transmissionand branching of E-W transmission, while the optical coupling/branchingunits shown in FIG. 1 are used one for the W-E transmission and E-Wtransmission respectively.

Specifically, the wavelength-multiplexed signal light (λ2-λ16) amplifiedby the receiving optical amplifier 3R of W-E transmission passes throughthe DCF 22 which is used for the wavelengths passing through the Centerstation, thereby undergoing the dispersion compensation. The multiplexedsignal light having passed through the DCF 22 and the signal light ofwavelength λ1 having been transmitted from the optical transmission unit70 and having passed through the DCF 23 are multiplexed by the coupler5, amplified by the transmission optical amplifier 3T, and transmittedover the transmission path 7. The wavelength-multiplexed signal (λ1-λ16)amplified by the receiving optical amplifier 3R of E-W transmissionpasses through the DCF 21 which is used for the wavelengths passingthrough the Center station and the wavelength to be split, thereby undergoing the dispersion compensation. The optical branching filter 4extracts the signal light of wavelength λ1, which is then transmitted tothe optical receiving unit 90. The remaining multiplexed signal (λ2-λ16)is given second dispersion compensation by the DCF 22 which is used forwavelengths passing through the Center station, amplified by thetransmission optical amplifier 3T, and then sent out over thetransmission path 7.

The DCF 22 and DCF 23 for W-E transmission have their dispersion valuesselected to be −1100 and −400 ps/nm, respectively, and the DCF 21 andDCF 22 for E-W transmission have their dispersion values selected to be−600 and −500 ps/nm, respectively. Consequently, the split wavelength,the wavelengths passing through the optical coupling/branching unit 100,and the coupled wavelength can have dispersion values of −600 ps/nm,−1100 ps/nm, and −400 ps/nm, respectively, as in the case of theembodiment shown in FIG. 1.

In this embodiment, the Center station can perform the high-speedcommunication at 10 Gbits/s only with the East station but not with theWest station. This system performance is sufficient for some operation,and is advantageous in having only one optical coupling/branching unit.In this case, when the Center station develops a need of communicationwith the West station, it is equipped with another opticalcoupling/branching unit 100′ for the communication with the Weststation, and the DCFs are replaced appropriately to match with thetransmission path.

This embodiment is useful for constructing a transmission system whichmatches with a demanded type of communication, while minimizing theinitial construction cost.

An optical transmission unit which is an embodiment of this inventionwill be explained with reference to the block diagram of FIG. 4 and alsoin connection with FIG. 7.

A wavelength-multiplexed signal light (λ1-λ16) inputted over thetransmission path 7 is amplified by a receiving optical amplifier 3R,and then passes through a DCF 21 which is provided for wavelengths λ1-λ4to be split. The multiplexed signal light (λ1-λ16) which is weakened byhaving passed through the DCF 21 is amplified by an optical amplifier32. Signal lights of wavelengths λ1-λ4 are split out of the amplifiedsignal light (λ1-λ16) by an optical branching filter 4 and outputted tothe outside. The remaining multiplexed signal light (λ5-λ16) passesthrough a DCF 22 which is provided for pass-through in consideration ofdispersion by the DCF 21. The multiplexed signal light (λ5-λ16) isamplified again by an optical amplifier 33 to regain the light levelwhich has been weakened by the branching filter 4 and DCF 22.

Signal lights of wavelengths λ1′-λ4′ (λn′=λn, n=1-4) which are inputtedfrom the outside are amplified by optical amplifiers 37 so as to havetheir light levels adjusted individually. The signal lights of λ1′-λ4′are next multiplexed by an optical coupler 5, and the resultingwavelength-multiplexed signal light (λ1′-λ4′) passes through a DCF 23which is provided for coupling. The multiplexed signal light (λ1′-λ4′)which has been weakened by the optical coupler 5 and DCF 23 is amplifiedby an optical amplifier 34. The amplified multiplexed signal light(λ1′-λ4′) is multiplexed with the multiplexed signal light (λ5-λ16) byan optical coupler 5. The resulting wavelength-multiplexed signal light(λ1′-λ4′ plus λ5-λ16) is amplified by a transmission optical amplifier3T, and then outputted over the transmission path 7.

In regard to the parameters of devices used in this embodiment, the DCF21 has a dispersion value of −600 ps/nm and a loss of 5.0 dB, the DCF 22has a dispersion value of −500 ps/nm and a loss of 4.5 dB, the DCF 23has a dispersion value of −400 ps/nm and a loss of 4.0 dB, the opticalbranching filter 4 has a loss of 3.5 dB, and the optical coupler 5 has aloss of 3.0 dB.

The receiving optical amplifier 3R is excited by a pumping light of 120mW to have a gain of 21.0 dB, the optical amplifiers 32, 33 and 34 areexcited by a pumping light of 25 mW to have a gain of around 11.0 dB,the optical amplifier 37 is excited by a pumping light of 25 mW to havea gain of around 6.0 dB, and the transmission optical amplifier 3T isexcited by a pumping light of 150 mW to have a gain of 17.0 dB. Theoptical amplifiers 32, 33 and 34 may be excited by a pumping light of 50mW.

Although in this embodiment, the signal light which is weakened in levelby the DCFs, branching filter and coupler is amplified by three opticalamplifiers, it can be treated by only one optical amplifier 32 havingits erbium-doped optical fiber selected in length appropriately andbeing exciting with a pumping light of 150 mW. Alternatively, the signallight can be treated by two optical amplifiers 32 and 34 excited at 100mW and 25 mW, respectively. However, using a number of low-excitationoptical amplifiers will be less expensive to achieve a specificperformance since pumping laser diodes are expensive increasingly astheir power rating rises.

Next, the detailed structure of the optical branching filter 4 will beexplained with reference to the block diagram of FIG. 7.

A transmitting wavelength-multiplexed signal light including wavelengthsλ1-λ16 passes through an optical circulator 45, and has its wavelengthsλ1 and λ2 reflected by black gratings 41 and 42, respectively. Theremaining multiplexed signal light of λ3-λ16 passes through anotheroptical circulator 46, and has its wavelengths λ3 and λ4 reflected in asimilar way. The remaining multiplexed signal light of λ5-λ16 is passedthrough the optical branching filter 4.

The optical circulators 45 and 46 are designed to conduct a light fromleft to right on the drawing, while rotating a right-to-left lightclockwise and outputting it to the third port. Accordingly, the signallights of λ1 and λ2 are directed to the optical branching filter 47 tobe separated in wavelength as above-mentioned way. The signal lights ofλ3 and λ4 are separated in a similar way.

By the way, the optical branching filters 47 and 48 may otherwise be thewaveguide type multiplexers. The optical branching filter 4 may be a WDM(Wavelength Division Multiplexer/Demultiplexer) of other kind.

According to this embodiment, it becomes possible to retain a highsignal light level by amplifying with optical amplifiers the signallights which are weakened by the DCFs, branching filter and coupler.Retaining a high signal light level prevents the deterioration of noisefigure for a long-distance transmission, and enables a long-distancetransmission, with optical couplers/branching filters being used.

FIG. 5 shows by block diagram an optical transmission unit which isanother embodiment of this invention. A wavelength-multiplexed signallight (λ1-λ16) inputted over the transmission path 7 is amplified by areceiving optical amplifier 3R and further amplified by an opticalamplifier 32, and fed to an optical branching filter 4′. A multiplexedsignal light of wavelengths λ1-λ4 split by the branching filter 4′passes through a DCF 21 which is used for λ1-λ14. The multiplexed signallight of λ1-λ4 passing through the DCF 21 is demultiplexed by abranching filter 4 into individual signal lights, and then amplified byoptical amplifiers 36 to regain their levels which have been weakened bythe DCF 21 and optical branching filter 4.

The remaining multiplexed signal light of λ5-λ16 which has passedthrough the optical branching filter 4′ passes through a DCF 22 which isprovided for pass-through. The multiplexed signal light of λ5-λ16 isamplified by optical amplifiers 33 to regain the level which has beenweakened by the optical branching filter 4′ and DCF 22.

Signal lights of λ1′-λ4′ inputted from the outside are amplified byoptical amplifiers 37 so as to have their light levels adjustedindividually. The signal lights of λ1′-λ4′ are multiplexed by an opticalcoupler 5, and then passes through a DCF 23 which is provided for thewavelength multiplexed signal lights (λ1′-λ4′). The multiplexed signallight (λ1′-λ4′) is amplified by an optical amplifier 34 to regain thelevel which has been weakened by the coupler 5 and DCF 23. The amplifiedwavelength multiplexed signal light (λ1′-λ4′) is multiplexed with thewavelength multiplexed signal light (λ5-λ16) by an optical coupler 5.The resulting multiplexed signal light (λ1′-λ4′ plus λ5-λ16) isamplified by a transmission optical amplifier 3T, and then outputtedover the transmission path 7.

In regard to the parameters of devices used in this embodiment, the DCF21 has a dispersion value of −600 ps/nm and a loss of 5.0 dB, the DCF 22has a dispersion value of −1100 ps/nm and a loss of 9.5 dB, the DCF 23has a dispersion value of −400 ps/nm and a loss of 4.0 dB, the opticalbranching filter 4 has a loss of 3.0 dB, and the optical coupler 5 has aloss of 3.0 dB. The receiving optical amplifier 3R is excited by apumping light of 120 mW to have a gain of 21.0 dB, the opticalamplifiers 32, 33 and 34 are excited by a pumping light of 25 mW to havea gain of around 11.0 dB, the optical amplifier 37 is excited by apumping light of 25 mW to have a gain of around 6.0 dB, and thetransmission optical amplifier 3T is excited by a pumping light of 150mW to have a gain of 17.0 dB.

According to this embodiment, it becomes possible to retain a highsignal light level by amplifying with optical amplifiers the signallights which are weakened by the DCFs, branching filter and coupler,thereby. Retaining a high signal light level prevents the deteriorationof noise figure for a long-distance transmission, and enables along-distance transmission, with optical coupling/branching units beingused.

FIG. 6 shows by block diagram an optical transmission unit which isstill another embodiment of this invention, and is basically the same instructure as the unit shown in FIG. 5. The same portions are referred bythe common reference numerals as the above-mentioned embodiments.

This embodiment is characterized by the provision of an optical coupler5 between the optical branching filter 4′ and DCF 22, so that part ofthe wavelengths which have been split by the branching filter 4′ iscoupled again and treated to pass through the optical transmission unit.This arrangement is intended for expectation of an increased trafficvolume of the place where the optical transmission unit is installed.The optical transmission unit is initially used for the band oftransmission between other places, and when the volume of transmissionof this place increases, it can be used for the transmission betweenthis place and other place. This embodiment effects the capability ofvarying the number of wavelengths depending on the data traffic volumeof the place where the transmission unit is installed.

FIG. 8 shows by block diagram a bidirectional optical transmissionsystem based on another embodiment of this invention. This system ischaracterized by including a number of Center stations, in contrast tothe system of FIG. 1 which includes only one Center station.

The wavelength-multiplexed signal (λ1-λ16) amplified by the receivingoptical amplifier 3R for W-E transmission equipped in Center station 1first passes through the DCF 21. The signal next passes through theoptical branching filter 4, which splits the light of wavelength λ1 outof the multiplexed signal. The remaining multiplexed signal (λ2-λ16)passes through the DCF 22 so that it is rendered the dispersioncompensation. The wavelength-multiplexed light which has passed the DCF22 and the signal light of wavelength λ1 transmitted from the opticaltransmission unit 70 and passing through the DCF 23 are multiplexed bythe optical coupler 5, amplified by the transmission optical amplifier3T, and outputted to the transmission path 7. The same arrangement isequipped for E-W transmission.

The wavelength-multiplexed signal (λ1-λ16) transmitted from the Centerstation 1 is amplified by the receiving optical amplifier 3R for W-Etransmission equipped in Center station 2, and it first passes throughthe DCF 24. The signal next passes through the optical branching filter4, which splits the light of wavelength λ2 out of the multiplexedsignal. The remaining multiplexed signal (λ1, λ3-16) passes through theDCF 25 so that it is rendered the dispersion compensation. Thewavelength-multiplexed light which has passed the DCF 25 and the signallight of wavelength λ2 transmitted from the optical transmission unit 70and passing through the DCF 26 are multiplexed by the optical coupler 5,amplified by the transmission optical amplifier 3T, and outputted to thetransmission path 7. The same arrangement is equipped for E-Wtransmission.

The wavelength-multiplexed signal (λ1-λ16) transmitted from the Centerstation 2 is amplified by the receiving optical amplifier 3R for W-Etransmission equipped in Center station 3, and it first passes throughthe DCF 27. The signal next passes through the optical branching filter4, which splits the light of wavelength λ2 out of the multiplexedsignal. The remaining multiplexed signal (λ1, λ3-λ16) passes through theDCF 28 so that it is rendered the dispersion compensation. Thewavelength-multiplexed light which has passed the DCF 28 and the signallight of wavelength λ2 transmitted from the optical transmission unit 70and passing through the DCF 29 are multiplexed by the optical coupler 5,amplified by the transmission optical amplifier 3T, and outputted to thetransmission path 7. The same arrangement is equipped for E-Wtransmission.

The DCFs have their dispersion values determined appropriately dependingon the dispersion values of the transmission path for receiving and thatfor transmission. In this example, dispersion values for thecompensation of wavelengths are set to be: DCF21: −600 ps/nm, DCF22:−500 ps/nm, DCF23: −400 ps/nm, DCF24: −300 ps/nm, DCF25: −250 ps/nm,DCF26: −200 ps/nm, DCF27: −300 ps/nm, DCF28: −500 ps/nm, DCF29: −400ps/nm.

This system can accomplish the high-speed communication at 10 Gbits/sbetween the West or East station and a number of center stations. By useof this system, the high-speed transmission at 10 Gbits/s among a numberof Center stations can be accomplished also.

FIG. 8 shows by block diagram a bidirectional optical transmissionsystem based on another embodiment of this invention. This system ischaracterized by including a number of Center stations, in contrast tothe system of FIG. 1 which includes only one Center station.

The wavelength-multiplexed signal (λ1-λ16) amplified by the receivingoptical amplifier 3R for W-E transmission equipped in Center station 1first passes through the DCF 21. The signal next passes through theoptical branching filter 4, which splits the light of wavelength λ1 outof the multiplexed signal. The remaining multiplexed signal (λ2-λ16)passes through the DCF 22 so that it is rendered the dispersioncompensation. The wavelength-multiplexed light which has passed the DCF22 and the signal light of wavelength λ1 transmitted from the opticaltransmission unit 70 and passing through the DCF 23 are multiplexed bythe optical coupler 5, amplified by the transmission optical amplifier3T, and outputted to the transmission path 7. The same arrangement isequipped for E-W transmission.

The wavelength-multiplexed signal (λ1-λ16) transmitted from the Centerstation 1 is amplified by the receiving optical amplifier 3R for W-Etransmission equipped in Center station 2, and it first passes throughthe DCF 24. The signal next passes through the optical branching filter4, which splits the light of wavelength λ2 out of the multiplexedsignal. The remaining multiplexed signal (λ1, λ3-16) passes through theDCF 25 so that it is rendered the dispersion compensation. Thewavelength-multiplexed light which has passed the DCF 25 and the signallight of wavelength λ2 transmitted from the optical transmission unit 70and passing through the DCF 26 are multiplexed by the optical coupler 5,amplified by the transmission optical amplifier 3T, and outputted to thetransmission path 7. The same arrangement is equipped for E-Wtransmission.

The wavelength-multiplexed signal (λ1-λ16) transmitted from the Centerstation 2 is amplified by the receiving optical amplifier 3R for W-Etransmission equipped in Center station 3, and it first passes throughthe DCF 27. The signal next passes through the optical branching filter4, which splits the light of wavelength λ2 out of the multiplexedsignal. The remaining multiplexed signal (λ1, λ3-λ16) passes through theDCF 28 so that it is rendered the dispersion compensation. Thewavelength-multiplexed light which has passed the DCF 28 and the signallight of wavelength λ2 transmitted from the optical transmission unit 70and passing through the DCF 29 are multiplexed by the optical coupler 5,amplified by the transmission optical amplifier 3T, and outputted to thetransmission path 7. The same arrangement is equipped for E-Wtransmission.

The DCFs have their dispersion values determined appropriately dependingon the dispersion values of the transmission path for receiving and thatfor transmission. In this example, dispersion values for thecompensation of wavelengths are set to be: DCF21: −600 ps/nm, DCF22:−500 ps/nm, DCF23: −400 ps/nm, DCF24: −300 ps/nm, DCF25: −250 ps/nm,DCF26: −200 ps/nm, DCF27: −300 ps/nm, DCF28: −500 ps/nm, DCF29: −400ps/nm.

This system can accomplish the high-speed communication at 10 Gbits/sbetween the West or East station and a number of center stations. By useof this system, the high-speed transmission at 10 Gbits/s among a numberof Center stations can be accomplished also.

As described above, the present invention accomplishes an opticalcoupler/branching filter which is capable of setting a proper dispersioncompensation value depending on the state of transmission path.Moreover, it becomes possible to compensate the loss of signal lightcaused by using optical coupler/branching filters and DCFs together bymeans of a plurality of low-excitation optical amplifiers, and therebyenable a long-distance transmission.

It is further understood by those skilled in the art that the foregoingdescription is a preferred embodiment of the disclosed device and thatvarious changes and modifications may be made in the invention withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. An optical transmission unit comprising: a firstdispersion compensator which compensates dispersion of a firstwavelength-multiplexed signal light, a first optical amplifier whichamplifies an output of said first dispersion compensator, an opticalbranching filter which splits the first wavelength-multiplexed signallight which has been amplified by said first optical amplifier into atleast a second wavelength-multiplexed signal light and a first signallight, a second dispersion compensator which compensates dispersion ofthe second wavelength-multiplexed signal light, and a second opticalamplifier which amplifies an output of said second dispersioncompensator.
 2. An optical transmission unit according to claim 1,wherein said first and second optical amplifiers include pumping laserdiodes having inputs of 50 mW or less.
 3. An optical transmission unitcomprising: a first dispersion compensator which compensates dispersionof a first wavelength-multiplexed signal light, a first opticalamplifier which amplifies an output of said first dispersioncompensator, a second dispersion compensator which compensatesdispersion of a first signal light, a second optical amplifier whichamplifies an output of said second dispersion compensator, and anoptical coupler which couples an output of said second optical amplifierto an output of said first optical amplifier.
 4. An optical transmissionunit according to claim 3, wherein said first and second opticalamplifiers include pumping laser diodes having inputs of 50 mW or less.5. An optical coupling/branching unit comprising: an optical branchingfilter which receives a first wavelength-multiplexed signal light whichis an output of a first dispersion compensator, outputs a signal lightof a first wavelength to an optical receiving unit, and outputs a secondwavelength-multiplexed signal light to a second dispersion compensator,and an optical coupler which receives and couples by wavelengthmultiplexing an output of said second dispersion compensator and anoutput of a third dispersion compensator and outputs a thirdwavelength-multiplexed signal light.
 6. An optical transmission systemcomprising a first terminal station which transmits a firstwavelength-multiplexed signal light at about 10 Gbits/s; a repeaterstation which receives the first wavelength-multiplexed signal light andtransmits a third wavelength-multiplexed signal light; and a secondterminal station which receives the third wavelength-multiplexed signallight; said repeater station including: a first dispersion compensatorwhich compensates the dispersion of the first wavelength-multiplexedsignal light, an optical receiver which receives a first signal lightincluded in the first wavelength-multiplexed signal light whosedispersion has been compensated, a second dispersion compensator, anoptical branching filter which splits a signal light of a firstwavelength from the first wavelength-multiplexed signal light whosedispersion has been compensated, transmits the split signal light tosaid optical receiver and transmits a second wavelength-multiplexedsignal light to said second dispersion compensator, an opticaltransmitter which transmits a second signal light of the firstwavelength, a third dispersion compensator which compensates dispersionof the second signal light from said optical transmitter, and an opticalcoupler which receives an output of said second dispersion compensatorand an output of said third dispersion compensator and transmits thethird wavelength-multiplexed signal light.